If the WHO had proven these in China, and you have no other alternative, it make sense to do both 1. buy tests you know are in active use and 2. make your own tests if needed.
Someone at the CDC must have been asleep or was ignored. China was shut down for weeks, the port of LA didnt have 25% of its normal volume in February and the WHO was shipping tests to over 60 countries _at the end of February_.
EDIT#1: I also emailed Dr Fauci a full week before about the short sightedness of allowing cruise ships to continue operating and while he was claiming "if you are healthy, they are fine". His office returned my mail a few days ago with a blanket "Dr Fauci is busy, and appreciates your comments" email.
EDIT#2: There is a more nuanced version of this discussion in the comments below. It is technically not correct to say the "WHO offered the US tests", the more technical distinction would be the "WHO supported leading research from Berlin, and the CDC did not. This decision lead to a delay in tests that did not have a higher number of false positives... (etc). The US generally has resources to produce these tests, and would not rely on the WHO." Either way, a test existed and the US could have chosen to take a more conservative stance by adopting the research and producing its own simulatneously. The Chinese economy was shutdown.
[1] https://www.politico.com/news/2020/03/06/coronavirus-testing...